UNIT-II - TIMBER

Timber Definition: Timber denotes wood which is suitable for building or carpentry and for various engineering and other purposes.

WOOD: The organic matter obtained from trees is called wood.

Classification of TIMBER Timber is classified on the basis of the following properties Mode of growth Durability Modulus of elasticity Grading Availability

Classification based on mode of growth Timber Exogenous Conifers Deciduous Endogenous

Types of timber (mode of growth ): Timber are classified into two groups. 1. Exogenous tress : The trees which grow in out wards across horizontal section of stem are called exogenous trees. These trees are only fit for engineering construction. 2. Endogenous trees: The trees which grow inwards in longitudinal fibrous mass are called endogenous trees.

Examples: Deodar Pine Chir Kail,etc Exogenous trees are again subdivided in to two types. 1.Conifers or Evergreen: They give soft wood. They have cone - shaped leaves. Examples: Deodar Pine Chir Kail,etc

Examples: Teak Sal etc. 2.Deciduous or Broad leaf Trees: These have hard wood. These have broad leaves. Examples: Teak Sal etc.

Comparison of softwood and hard wood S.No. Item Soft woods Hard woods 1. 2. 3. 4. 5. 6. Annual Rings Color Density Fire resistance Weight Strength Distinct Light Low Poor strong for direct pull & weak for equal resisting thrust Indistinct Dark High More Heavy strong for resisting tension, compression & shear

Structure of a Tree

Structure of tree: From the visibility aspect, the structure of a tree can be divided into two categories 1. Macro structure 2. Micro structure 1. Macro structure: The structure of wood visible to the naked eye or at a small magnification is called macro structure. The following figure shows the macro structure of exogenous tree.

CROSS SECTION OF AN EXOGENEOUS TREE

COMPONENTS OF AN EXOGENEOUS TREE (a) Pith: The innermost central portion or core of the tree is called pith or medulla. (b) Heart wood: The inner annual rings surrounding the pith is known as heart wood. (c) Sap wood: The cuter annual rings between heart wood and cambium layer is known as sap wood. (d) Cambium layer: Thin layer of sap between sap wood and inner bark is known as cambium layer. (e)Inner bark: The inner skin or layer covering the cambium layer is known as inner bark. (f) Outer Bark: The outer skin or cover of the tree is known as outer bark. (g) Medullary rays: The thin radial fibers extending from pith to cambium layer are known as medullary rays.

2. Micro structure: The structure of wood apparent only at great magnifications is called micro structure. Conductive cells Mechanical cells Storage cells

2) Classification based on DURABILITY Classification based on durability test conducted on specimens of size 600x50x50 mm . The specimens are buried in the ground up to half their length and are observed over several years . The forest research institute of India classified trees into three categories High durability : average life of the tree is 10 years Moderate durability : average life of the tree is 5 years Low durability : average life of the tree is less than 5 years

3) Classification based on GRADING IS 190-1974 & IS 1326-1976 Timbers are classified into 3 groups Special grade Grade I Grade II

4)Classification based on MODULUS OF ELASTICITY The MODULUS OF ELASTICITY E is determined by conducting the bending test on a standard specimen. Timber on this basis, is classified as: Group A: If E> 12.5 kN/mm2 Group B: If E=9.8-12.5 kN/mm2 Group C: If E=5.6-9.8 kN/mm2 5)Classification based on Availability IS 339-1963 Timbers are classified as: X: Most common-1000 Tonnes or more per year Y: common -250 -1000 Tonnes per year Z:Less common <250 Tonnes per year

CHARACTERISTICS OF GOOD TIMBERS Elasticity: The timber returns to its original shape when load causing its deformation is removed. Fire resistance: A dense wood offers good resistance to fire. Hardness: A good timber should be hard. Mechanical wear: A good timber should not deteriorate easily due to mechanical wear or abrasion.

Shape: A good timber should be capable of retaining its shape during conversion or seasoning Smell: A good timber should have sweet smell. Unpleasant smell indicates decayed timber Sound: A good timber should give a clear ringing sound when struck Strength: A good timber should be sufficiently strong for working as structural member such as joist, beam, rafter etc.

DEFECTS IN TIMBER: Most common defects in timber are: Improper seasoning and conversion Attack by fungi and insects. Natural forces: Heart Shakes Star Shakes Cup Shakes Radial Shakes Rind Galls Wind Cracks Knots Dead Wood

Heart Shakes: These are splits occurring in the centre of the tree and running from the pith (inner most part) to wards the sap wood from the medullary (vascular tissues) rays. In some timbers, these splits are hardly visible and in some timbers these are quite permanent. Heart shakes are caused due to shrinkage of interior parts due to age. A heart shake straight across the trunk is not a serious defect.

(2) Star Shakes: These are splits which radiate from the centre of the timber or from the bark (outer side), running in the planes of medullary rays. These occur due to severe frost or scorching heat of the sun.

(3) Cup Shakes: These are curved splits which separate the whole or part of one annual ring from an other. These are caused due to the unequal growth of the timber.

(4) Radial Shakes: These are similar to the star shakes and occur in felled timber when exposed to the sun during seasoning. Radial shakes are generally irregular, fine and numerous. In this many splits are appeared.

(5) Rind Galls: These are typical enlarged swellings and occur due to branches cut-off.

(6) Wind Cracks: These are shakes or splits on the sides of a bark of timber due to shrinkage of exterior surface exposed to atmospheric influence.

(7) Knots: These are the roots of the small branches of the tree. These are not harmful.

(8) Dead Wood: It is the deficient in strength and weight and is the result of trees being felled after maturity.

SEASONING OF TIMBER: As fresh timber which is obtained from trees contains about 30 to 40 % sap or moisture. This sap is very harmful for the life of a timber. Therefore, it is necessary to remove that sap by applying some special methods. All those methods which are used for removing the sap from timber are collectively termed as seasoning of timber.

Advantages of seasoned timber: It has reduced weight, It is strong and durable, It has resistance to decay or rot, It takes high polish, It is easier to work, Its life is more.

Types of Timber Seasoning: The main types of timber seasoning are as under. Natural Seasoning Water Seasoning AIR Seasoning (2) Artificial Seasoning (a)Kiln Seasoning, (b) Chemical Seasoning, (c) Electric Seasoning, (

Natural Seasoning: In the air seasoning or natural seasoning or natural drying, seasoning of timber, timber is dried by direct action of air, wind and sun. In this method, the timber logs are arranged one over the other, keeping some space or distance between them for air circulation of fresh air. Generally this type of seasoning requires few months to over a year, this is very slow process.

(2) ARTIFICIAL SEASONING: Kiln Seasoning, (b) Chemical Seasoning, (c) Electric Seasoning,

Kiln Seasoning: In kiln seasoning timber is placed in a chamber with some special heating arrangement. In this process one thing should be kept in mind that heating system should be under control, other wise timber will be crack or wrap . The time required for this seasoning is 3 to 12 days. This is quick process.

(b) Chemical Seasoning: In chemical seasoning carbon dioxide, ammonium carbonate or urea are used as agents for seasoning, those are applied in dry state, the inter surface of timber dries first than outer side. This ensures uniform seasoning. The time required for this seasoning is 30 to 40 days.

(c) Electric Seasoning: In this method electric current is passed through the timber logs. The time required for this seasoning is 05 to 08 hours.

(3) Water Seasoning: In water seasoning, timber logs are kept immersed whole in the flowing water. The sap present in timber is washed away. After that logs are taken out from water and are kept in open air, so water present in timber would be dried by air. The time required for this type of seasoning is 2 to 4 weeks.

DECAY OF TIMBER The main causes for decay are: Fungi attack Attack by insects Alternate dry and wet condition Use of timber without proper seasoning Shocks received by trees during growth

Preservation of Timber It means protecting from fungi and insects attack so that its life is increased. The following are the widely used: 1. Tar 2. Paints 3. Chemical salt 4. Creosote 5. ASCU

1. Tar Treatment: Hot coal tar is applied to timber with brush 1. Tar Treatment: Hot coal tar is applied to timber with brush. The coating of tar protects the timber from the attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage Appearance is not good after tar is applied

Paint improves the appearance of the timber. 2. Paints Treatment: Two to three coats of oil paints are applied on clean surface of wood. The paint protects the timber from moisture. The paint is to be applied from time to time. Paint improves the appearance of the timber. Solignum paint is a special paint which protects the timber.

3. Chemical salt Treatment: These are the preservatives made by dissolving salts in water. The salts used are copper sulphate, masonry chloride, zinc chloride and sodium fluoride. After treating the timber with these chemical salt paints and varnishes can be applied to get good appearance.

4. Creosote oil treatment: Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in an air tight chamber and air is exhausted. Then creosote oil is pumped into the chamber at a pressure of 0.8 to 1.0 N/mm2 at a temperature of 50°C. After 1 to 2 hours timber is taken out of the chamber.

5. ASCO Treatment: This preservative is developed by the Forest Research Institute, Dehradun. It consists of 1 part by weight of hydrated arsenic pent-oxide (As2O5, 2 H2O), 3 parts by Tonnes or more per year weight of copper sulphate(CuSO4⋅5 H2O) and 4 parts by weight of potassium dichromate (K2Cr2O7) or so Tonnes or more per year dium dichromate (Na2Cr2O7⋅2 H2O). This preservative is available in powder form. By mixing six parts of this powder with 100 parts of water, the solution is prepared. The solution is then sprayed over the surface of timber. This treatment prevents attack from termites. The surface may be painted to get desired appearance.

Tests on Timber Types of laboratory are: to find the phsical properties of timber Moisture content Tensile strength Compressive strength along grains Compressive strength across grains Bending strength Shear strength Hardiness strength Cleavability Torsional strength

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